The phosphorus cycle is the process that moves phosphorus through Earth's solid layers, water parts, and living parts. Unlike other cycles, the atmosphere does not play a major role in this process because phosphorus rarely becomes a gas. Phosphine, a rare gas form of phosphorus, is only created under special conditions. Because of this, scientists mostly study orthophosphate (PO₃⁻⁴), the most common form of phosphorus in nature, as it moves through land and water ecosystems.
Living things need phosphorus to function properly. It is a key part of DNA, RNA, ATP, and other important molecules. Phosphorus is also found in phospholipids, which make up cell membranes. Plants take in phosphorus as phosphate and use it to create organic compounds. In animals, inorganic phosphorus in the form of apatite (Ca₅(PO₄)₃(OH,F)) is a major part of bones and teeth.
On land, phosphorus becomes less available to plants over thousands of years because it is slowly carried away by runoff. When soil has low phosphorus levels, plant growth and the activity of soil microorganisms slow down, as shown in studies of soil microorganisms. These microorganisms can both store and release phosphorus in the cycle. In the short term, phosphorus changes through chemical, biological, or microbiological processes. Over long periods, however, tectonic movements and the weathering of phosphate-containing rocks, such as apatite, are the main ways phosphorus moves globally.
Phosphorus is often a limiting nutrient in water ecosystems, meaning it can restrict growth. However, when too much phosphorus enters water systems, it can cause excessive plant and algae growth, a process called eutrophication, which can happen in both freshwater and saltwater environments.
Human activities have greatly changed the global phosphorus cycle, mainly through mining and using phosphorus in fertilizers and industrial products. Some phosphorus is also lost as waste during mining and industrial processes.
Phosphorus in the environment
Phosphorus is a necessary nutrient for plants and animals. In water ecosystems, phosphorus is a limiting nutrient, meaning it is important for growth but not always available in enough amounts. Phosphorus is part of molecules that help living things survive and is found throughout the biosphere. Small amounts of phosphorus enter the atmosphere when dust containing phosphorus dissolves in rainwater or sea spray, but most phosphorus stays on land and in rocks and soil. Phosphates, found in fertilizers, sewage, and detergents, can pollute lakes and streams. Too much phosphate in freshwater or coastal marine water can cause large algae blooms. In freshwater, when these blooms die and decay, it leads to eutrophication. An example of this is the Canadian Experimental Lakes Area.
Freshwater algae blooms are usually caused by too much phosphorus, while saltwater blooms often happen when too much nitrogen is added. However, eutrophication can occur in both freshwater and saltwater if phosphorus levels rise sharply.
Phosphorus is most common in nature as part of the orthophosphate ion (PO₄³⁻), made of one phosphorus atom and four oxygen atoms. On land, phosphorus is found in rocks and minerals. Phosphorus-rich deposits often form in the ocean or from guano (bird droppings), and over time, geological processes bring ocean sediments to land. Weathering of rocks and minerals releases phosphorus in a form that plants can use, where it becomes part of organic compounds. Plants may be eaten by herbivores, and phosphorus is either stored in their bodies or excreted. When plants or animals die, phosphorus returns to the soil, where much of it becomes insoluble. Some phosphorus is carried back to the ocean by runoff. Over thousands of years, soils may become low in phosphorus, leading to ecosystem decline.
In freshwater ecosystems, phosphorus exists in four main forms: dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP), and particulate organic phosphorus (POP). Dissolved material passes through a 0.45 μm filter. DIP mainly includes orthophosphate (PO₄³⁻) and polyphosphate, while DOP includes substances like DNA and phosphoproteins. Particulate matter does not pass through the filter and includes living and dead organisms (POP) or minerals like hydroxyapatite (PIP). Inorganic phosphorus is soluble, while particulate organic phosphorus is insoluble and found in living and dead cells. Dissolved organic phosphorus comes from the breakdown of particulate organic phosphorus and is soluble.
Phosphates are important in biological processes, such as forming nucleotides, which store energy in cells (ATP) or build DNA and RNA. The structure of DNA relies on phosphate bonds. Phosphorus is also found in bones and teeth, where it strengthens them through calcium phosphate (hydroxyapatite). It is present in insect exoskeletons and phospholipids in cell membranes. Phosphorus helps maintain acid-base balance in the human body.
Phosphates move quickly through plants and animals, but processes that move phosphorus through soil or oceans are very slow, making the phosphorus cycle one of the slowest biogeochemical cycles.
The global phosphorus cycle includes four major processes:
On land, phosphorus becomes available to plants and animals when phosphorus-containing rocks weather. Apatite, the most common phosphorus mineral in Earth’s crust, dissolves through chemical reactions or erosion. Dissolved phosphorus is used by plants and returned to soil after they decay. Soil minerals (like iron and aluminum compounds) often trap phosphorus, reducing its availability. Plants and microorganisms use special strategies to absorb phosphorus from low concentrations.
Soil phosphorus is carried to rivers and lakes, where it may settle in lake sediments or flow into the ocean. Atmospheric phosphorus from dust or rain also enters the ocean. In seawater, phytoplankton absorb dissolved inorganic phosphorus (orthophosphate) and convert it into organic compounds. When phytoplankton die, phosphorus is released back into the water. Some organic phosphorus is broken down by bacteria and reused. Most phosphorus is recycled in the water, but about 1% is buried in ocean sediments. Processes in sediments can release phosphorus back into the water near the ocean floor. These processes are similar to those in lakes and rivers.
Although orthophosphate (PO₄³⁻), the main inorganic phosphorus form, has an oxidation state of +5, some microorganisms can use phosphonate or phosphite (oxidation state +3) by converting them to orthophosphate. Recent discoveries show that reduced phosphorus compounds may play a role in oceanic phosphorus cycles.
Phosphorus availability depends on how quickly it is released during rock weathering. Apatite dissolution controls ecosystem productivity. Apatite [Ca₅(PO₄)₃OH] undergoes chemical reactions that release phosphorus. Most released phosphorus reacts with soil minerals and becomes unavailable to organisms over time. Available phosphorus is found in the upper soil layers, while deeper phosphorus is involved in chemical reactions with minerals. Plants rely on quickly absorbing phosphorus from dead organic matter in the upper soil. Phosphorus is limited in supply for plant growth, and its movement through soil and oceans is slow, making the phosphorus cycle one of the slowest biogeochemical cycles.
Low-molecular-weight organic acids in soil come from microorganisms or plant roots. These acids can form stable compounds with metal ions, releasing inorganic phosphorus linked to aluminum and iron.
Human influences
Nutrients are vital for the growth and survival of living things and are necessary for creating and keeping healthy ecosystems. Humans have greatly changed the phosphorus cycle by mining phosphate rock. For thousands of years, phosphorus entered the environment mainly through the natural breakdown of phosphate-containing rocks, which slowly replaced phosphorus lost through processes like runoff. However, since the 1840s, when mining and extracting phosphorus became more common, about 110 teragrams of phosphorus have been added to the environment. This trend continues, as global phosphorus mining has increased 72 times since 1900, with an expected annual growth of 4%. Most of this mining is used to make fertilizers for crops worldwide. However, the rate of mining is faster than the Earth’s natural systems can replace the lost phosphorus. Scientists are studying ways to recycle phosphorus more effectively, such as using microorganisms. Overall, human activity has significantly altered the phosphorus cycle, affecting food security, water quality, and the availability of this nutrient.
Other human activities can harm the phosphorus cycle, such as applying too much liquid hog manure to crops. Using biosolids, which are treated organic waste, can also increase phosphorus levels in soil. In poorly drained areas or places where snowmelt causes waterlogging, low-oxygen conditions can develop within 7–10 days. This leads to a rapid rise in phosphorus levels in soil water, allowing phosphorus to move into groundwater. Additionally, low-oxygen conditions change phosphorus from stable forms to more mobile ones, increasing the risk of phosphorus loss. This is especially important for managing areas where agricultural waste disposal is already a challenge. Experts recommend considering soil water conditions when creating waste management rules to address these issues.